strain EBF65/65 harbours a cryptic plasmid, pAV2, which has been shown by electrophoretic separation on agarose gels to have a molecular mass of approximately 13.5 megadaltons (Md). Transfer of the previously described sex factor pAV1 (Hinchliffe & Vivian, 1980, ) from the hospital strain JC17 into strains possessing pAV2 occurs only at a low frequency, whereas transfer to similar strains lacking pAV2 occurs at a much higher frequency. In EBF65/65, pAV1 may be present in strains possessing or lacking pAV2; pAV1 strains lacking pAV2 correspond to strains previously described as pAV1a (Hinchliffe & Vivian, 1980) whereas pAV1 strains which also possess pAV2 correspond to pAV1b strains. The genetic evidence presented here is consistent with the hypothesis that pAV2 specifies a host restriction and modification system that is active against pAV1. Physical evidence from agarose gel electrophoresis indicates that pAV1 corresponds to a band of approximately 85 Md in strain JC17. The corresponding band in strains of EBF65/65 is difficult to distinguish because of the presence of a further cryptic plasmid band of approximately 88 Md, designated pAV3. A small cryptic plasmid of approximately 6 Md, designated pAV4, is reported for EBF65/65.


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